The present invention relates generally to an inspection system and an inspection method. More particularly, the present invention relates to an inspection system for and a method of determining errors introduced by a photolithographic camera or stepper unit.
The semiconductor or IC industry desires to manufacture integrated circuits (ICs) with higher and higher densities of devices on a smaller chip area to achieve greater functionality and to reduce manufacturing costs. This desire for large scale integration has led to a continued shrinking of circuit dimensions and device features.
The ability to reduce the size of structures, such as, gate lengths in field-effect transistors and the width of conductive lines, is driven by lithographic performance. In conventional commercial fabrication processes, lithographic systems, such as, photolithographic cameras or stepper units, expose a photoresist material to a pattern of radiation. The photoresist material is developed in accordance with the pattern of radiation to form a pattern of the photoresist material on a wafer. The wafer is processed in accordance with the pattern of photoresist material.
A conventional lithographic system or photolithographic machine can be a projection printing machine using refractive optics in a step-and-repeat projection method.
Conventional lithographic systems generally include a light source configured to provide radiation or light at one or more wavelengths. For example, the light source may include an excimer laser producing radiation at a wavelength of 248 nm, 193 nm, and/or 157 nm. The excimer laser can use a KrF source, a ArF source, a F2 source, etc. The lithographic systems can further include a first lens assembly, a Chromium (Cr) mask, and a second lens assembly. The radiation is provided from the light source through the first lens assembly, through the mask, through the second lens assembly to a semiconductor wafer having a layer of photoresist material.
The first lens assembly can be a condenser lens, and the second assembly can be an objective lens. The radiation can be light, such as ultraviolet light, vacuum ultraviolet (VUV) light, and deep ultraviolet light. In alternative systems, the radiation can be x-ray radiation, e-beam radiation, extreme ultraviolet (EUV) light, etc.
As described above, conventional lithographic systems can utilize multiple optical elements to focus and direct light to the semiconductor wafer. Generally, the multiple optical elements can be considered as a single equivalent lens. The pupil of the lithographic system refers to the equivalent lens. The size of the pupil is the diameter of the equivalent lens and the location of the pupil is the location of the plane of the equivalent lens. The pupil is utilized to mathematically model image formation by the optical elements of the lithographic system.
Conventional lithographic systems include lens assemblies which are susceptible to lens aberrations or errors. These errors result in errors in the wavefront that is used by the lithographic stepper unit to produce the image on the wafer. As light passes through the objective lens assembly, an imperfection can locally increase or decrease the finite optical path. These imperfections can result in placement errors or critical dimension (CD) errors in the lithographic pattern. These errors are particularly problematic as sizes of lithographic features become smaller.
Accordingly, the pupil of the conventional lithographic system is often tested to determine at which locations errors are introduced into the pupil plane. Heretofore, the pupil of the conventional lithographic systems are probed or tested before installation (e.g., off-line) of the lithographic system by a laser interferometer. The use of a laser interferometer is not practicable after the lithographic system is installed. Other conventional techniques probe particular aberrations and require overlay measurement tools.
Thus, there is a need for a highly accurate inspection system that can be utilized to detect defects and patterns on a pupil. Further, there is a need for a semiconductor fabrication inspection tool for measuring and locating wavefront errors associated with a lithographic system. Even further still, there is a need for a process or method of detecting pupil errors or lens aberrations in situ (e.g., in-line). Even further still, there is a need for an inspection tool and inspection method that is capable of reliably detecting errors on the entire pupil.
An exemplary embodiment relates to a method of inspecting a lens assembly for a lithographic stepper. The method includes providing radiation at a first coherence through a mask or reticle to a photoresist material, and providing radiation at a second coherence through the mask or reticle to the photoresist material. The method can also include developing the photoresist material, and observing the photoresist material.
Another exemplary embodiment relates to a method of inspecting a pupil associated with manufacture of an integrated circuit. The method includes providing a pattern of low coherence radiation to a photoresist material, providing a pattern of high coherence radiation to the photoresist material, and developing the photoresist material. The method also includes observing the photoresist material.
Still another exemplary embodiment relates to an inspection system for an optical system. The optical system is for use in an integrated circuit fabrication system. The inspection system includes means for providing radiation at a first coherence to a photoresist material, means for providing radiation at a second coherence to the photoresist material. The inspection system also includes means for developing the photoresist material and means for observing the photoresist material.